1. Field of the Invention
The present invention relates to optical time domain reflectometry (OTDR) methods and apparatus for carrying out such methods.
2. Description of the Prior Art
In OTDR light at a first wavelength (.lambda..sub.0) is launched into one end of an optical fibre and optical radiation back-scattered along the fibre is measured. In distributed sensing using OTDR the back-scattered radiation is used to measure respective values of a physical parameter at different locations along the fibre, which is deployed in a region of interest. In optical time domain reflectometers, which are used for characterising fibres in their production environments or in installed cables, the back-scattered optical radiation is used, for example, to locate faults in the fibre or to measure the attenuation characteristics of the fibre.
The back-scattered signals may result from either elastic or inelastic scattering processes. Rayleigh scattering produces elastically scattered signals with a wavelength distribution substantially the same as the injected signal (.lambda..sub.0). Brillouin and Raman scattering on the other hand are inelastic scattering processes which each produce pairs of spectral bands. Each pair of first order bands comprises one (the Stokes band) centred on a longer wavelength (.lambda..sub.+1) than that of the injected signal (.lambda..sub.0) and the other (the anti-Stokes band) centred on a shorter wavelength (.lambda..sub.-1) than that of the injected signal, such that the pair is centred on the injected signal wavelength. The spectrum would normally contain several successive orders (at wavelengths .lambda..sub..+-.n, where n=1, 2, 3 . . . ) resulting from a particular scattering process, the intensity of the bands decreasing as the order increases. In some silica-based materials the Raman spectrum contains more than one band of significant intensity, for example, in a binary P.sub.2 O.sub.5 .multidot.SiO.sub.2 glass, P.sub.2 O.sub.5 has a band around 1390 cm.sup..sup.-1 in addition to the main silica band at around 440 cm.sup.-1.
For an injected signal of 904 nm in silica, Brillouin anti-Strokes and Brillouin Strokes back-scattered signals are shifted by about 0.05 nm from the injected signal, and the first order Raman Stokes and Raman anti-Stokes back-scattered signals are shifted by about 34 nm. The wavelength shifts for the Brillouin and Raman scattered signals are respectively about 0.058 nm and 50 nm for a 1.06 .mu.m signal, and about 0.084 nm and 100 nm for a 1.53 .mu.m injected signal in silica.
The Brillouin and Raman back-scattered signals have intensities dependent on physical parameters, such as temperature. Typically for silica fibres at room temperature, the Raman Stokes and anti-Stokes signals are less intense than those of the Brillouin back-scattered signals, the Raman signals having first order intensities which are lower than the Rayleigh backscatter signal at 1.064 .mu.m by about 18 dB and 28 dB respectively, compared to the Brillouin signals which are about 13 to 16 dB lower than the Rayleigh backscatter signal. With 7 ns, 50W pulses at 1.06 .mu.m in an industry-standard multi .mu.mode fibre (50 .mu.m diameter core, 125 .mu.m cladding, graded-index core and a numerical aperture of 0.20), the power of Raman anti-Stokes wavelength light resulting from back-scattering near the receiving/injecting end of the fibre is about 50 nW.
In a known OTDR method of distributed sensing, such as that described in U.S. Pat. No. 4,823,166, a 1.5W modulated optical signal of wavelength 854 nm at 4 kHz and having a pulse width of 40 ns is injected into one end of an optical fibre of more than 1 km in length. A back-scattered signal is returned to the first end and comprises the aforementioned elastically and inelastically back-scattered signals which are then filtered to remove substantially all but the Raman anti-Stokes signal which passes to detecting means for measurement of its intensity, referenced to the total back-scatter signal. From the change in intensity with the elapsed time from the injected signal, the distribution of a particular physical parameter such as the temperature along the fibre may be deduced.
In an earlier known OTDR method, such as that described in GB-2140554, pulsed light is launched into one end of an optical fibre and back-scattered Raman Strokes and anti-Stokes signals are separated and measured. Ratios of the measurements are then obtained from which a temperature distribution for the fibre is derived.
In a further known OTDR method such as that described in U.S. Pat. No. 5,217,306, optical signals of wavelength 1.32 .mu.m from a source comprising a diode-pumped solid state laser are sent through a length of optical fibre with enhanced Raman scattering properties, an attenuator and an optical filter to emit therefrom a test signal of wavelength 1.40 .mu.m for injection into a sensing optical fibre for measuring temperature therealong. The conversion of the wavelength between 1.32 .mu.m and 1.40 .mu.m is achieved by Stimulated Raman Scattering (SRS) of the first wavelength to produce the second, longer wavelength. Raman anti-Stokes and Raman Stokes signals of respective wavelengths 1.32 .mu.m and 1.50 .mu.m which are subsequently back-scattered from positions along the sensing optical fibre are then detected and processed in the same way as the first mentioned OTDR method.
The range of test signals in optical fibres is limited by dispersion and attenuation. For a given fibre, therefore, the test signal is desirably selected to be at a wavelength corresponding to a minimum in the attenuation/dispersion characteristics of the fibre. For a fibre material such as GeO.sub.2 doped silica, a dispersion minimum for the material itself occurs at a wavelength of 1.3 .mu.m, and an attenuation coefficient minimum of about 0.2 dB/km at a wavelength .lambda.=1.55 .mu.m.
An OTDR distributed sensing system suitable for long range sensing, which uses injected wavelengths in the range 1.51 .mu.m to 1.59 .mu.m, is described in the applicant's British patent application no. 9307660.2 filed on Apr. 14, 1993, the disclosure of which is hereby incorporated by reference.
Long-range sensing requires high power sources. However, at high power, non-linear optical effects appear. A particular problem is stimulated Raman scattering which converts the wavelength launched into the fibre to the first order Stokes wavelength, mainly in the forward direction. As shown in FIGS. 1(A) and 1(B) of the accompanying drawings, which are graphs illustrating the variation in the intensity of optical radiation in an industry-standard single mode fibre (index difference 0.35%, cut-off wavelength 1200 nm) at a test wavelength .lambda..sub.0 (dashed line 200) of 1530 nm and the first Stokes Raman wavelength .lambda..sub.+1 (solid line 201) with distance along the fibre for launch powers of 1W and 3W respectively, the stimulated emission grows along the fibre, which for long-range sensing could be many kilometers in length, until eventually substantially all the light launched into the fibre is converted to the Stokes wavelength. The values given in FIGS. 1(A) and 1(B) are for a typical fibre and are dependent on the design of fibre and test wavelength used. The rate of growth is proportional to the intensity (power/area) of the light launched into the fibre and to parameters of the glass, and is therefore inversely proportional to the area over which the power is confined.
Owing to stimulated Raman scattering, in distributed sensors operating at sufficiently high power levels (e.g. for long-range applications), the signal at the Stokes wavelength becomes significantly greater than it would be without non-linear effects and this distorts the measurements where the Stokes signal is used as a reference. In addition, because power is lost from the light launched into the fibre through conversion to the Stokes wavelength, the signal at the anti-Stokes wavelength is correspondingly weakened. Furthermore, because of the significant build-up of the Stokes power in the forward direction, the connectors in the fibre reflect a large amount of this power, which the filters in the receiver transmit. The strong signal reflected from the connectors can therefore distort the preampllfier output over subsequent fibre sections. FIG. 2 of the accompanying drawings, which shows the back-scattered signals (normalised to unity at Okm) at the anti-Stokes (chain line 202) and Stokes (dashed line 203) wavelengths over a distance of 50,000 meters and the ratio of those two signals (solid lane 204) for an injected power of 3W at 1530 nm in a typical single-mode fibre, illustrates how the Stokes signal increases with distance along the fibre, eventually overcoming the normal effects of fibre attenuation, and thus distorts the anti-Stokes/Stokes ratio. FIG. 3 of the accompanying drawings compares the total forward travelling power at the Stokes wavelength (dashed line 205) when Raman gain is taken into account with that which has a purely linear (spontaneous) origin (solid line 206) in a typical single-mode fibre for an injected pulse of 1.0W at 1530 nm.
By way of example, for a non-dispersion-shifted single mode fibre having a core refractive index of 1.45 and a numerical aperture of 0.1, using an injected signal of 1.53 .mu.m at which it has an attenuation of 0.197 dB/km (0.292 dB/km at the anti-Stroke Raman wavelength of 1.43 .mu.m and 0.311 dB/km at the Stokes wavelength of 1.64 .mu.m), a 2% departure from the value for linear operation, for example, in the ratio of the anti-Stokes to Stokes signal gives an error of roughly 3.degree. C. in distributed temperature sensing applications. For such a fibre 50,000 meters in length, using an injected signal of wavelength 1.53 .mu.m, the non-linearity error begins to exceed 2% at a launch power between 0.9W and 1.0W, the 2% error being exceeded at a distance of 39,650 m at a power of 1W and at 12,050 m at a power of 2W. Although these values are very sensitive to a number of factors, including very small changes in fibre loss, they serve to illustrate the effect of stimulated Raman scattering on measurements taken using distributed sensing.
The early onset of stimulated Raman scattering can also be a problem in optical time domain reflectometers in which it is especially important to ensure that the optical fibre is operated in the linear regime, since the referencing used in distributed sensing is not available.
Thus, to avoid the onset of stimulated Raman scattering, heretofore the power of the light launched into the fibre has been restricted, with the result that the range over which measurements can be taken is limited.
Similar problems arise owing to stimulated Brillouin scattering.